Digital Equipment RW504-ZA, Service Manual

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During normal address translation, after the original physical address is found
via the slip sparing algorithm, the SDL is checked to see if that physical address
was ’spared’ through the replacement sparing algorithm. If so, the replacement
physical address is substituted for the original physical address.
In the event a sector needs to be replaced, i.e. due to a Reassign Blocks Command
or automatic reallocation during a write command, a new defect/replacement pair
will be added to the SDL (if the new defects not already in the SDL) or an
existing defect/replace will be updated (if the new defect is already in the SDL).
6.5.8 Error Thresholds
Although not directly related to disk format, the various error thresholds are the
basis for deciding whether or not to spare a sector. This could happen during the
certification process (i.e. the slip sparing algorithm) or auto-reallocation during
a SCSI Write command (i.e. the replacement sparing algorithm). These error
thresholds are related to the format of a sector in the user zone.
Each sector in the user zone consists of a header, user data, and error correction
information. The first error threshold of importance involves information in the
sector header. Each header consists of three copies of the sector’s track number,
sector number, and a Cyclic Redundancy Check (CRC). The number of these
‘‘sector IDs’’ is used as an error threshold.
The other error threshold of interest involves information about error correction.
The error correction information that follows the data is often referred to as ECC
(Error Correction Code) data. This data is a compact way to encrypt information
about the original data written to the disk. With this information a drive can
correct up to 8 bytes per data interleave in the User Data of a sector. (Consult
the ISO standard for more details.) The actual number of bytes per interleave
requiring correction is used as an error threshold. Table 6–3 shows the error
thresholds for the optical drive. The sector IDs column refers to the minimum
number of sector IDs that must be read correctly for the corresponding operation
to be deemed successful. The ECC level column refers to the maximum number
of bytes per interleave that require correction in order for the corresponding
operation to be deemed successful.
Theory of Operation 6–21